The invention relates to a catalyst deterioration detecting system for determining deterioration of catalysts located in an exhaust system of an internal-combustion engine, and more particularly, it relates to a system for detecting the degree of deterioration of respective catalysts in a catalyst converter that includes an upstream catalyst located immediately below the engine in the upstream side of the exhaust system and a downstream catalyst or a downstream catalyst located on the downstream side in the exhaust system.
In general, a three-way catalyst remains inactive until its temperature is raised by exhaust gas after the engine has started. Therefore, purification of the exhaust gas is not sufficiently performed immediately after the engine start.
As one of the conventional techniques for accelerating the purification of the exhaust gas just after the engine start, there is a known catalyst converter that disposes an upstream catalyst relatively near the engine in addition to a downstream catalyst. In such a catalyst converter that is provided with two catalysts in series, the temperature of the upstream catalyst rises more quickly as heated by the exhaust gas just after the engine start than that of the downstream catalyst. Accordingly, this type of catalyst converter can start the purification of the exhaust gas more quickly than another type of a converter that is provided with a downstream catalyst only.
As for the catalyst converter which has two catalysts in series, the upstream catalyst located on the upstream side generally deteriorates faster than the downstream catalyst located on the downstream side. However, thermal load of the downstream catalyst on the downstream side occasionally increases depending on the operational conditions, so that deterioration of the downstream catalyst on the downstream side may progress more quickly than the upstream catalyst.
A prior art for determining deterioration of each catalyst in the catalyst converter having two catalysts in series is described in Japanese Patent No. 3076417. The catalyst converter comprises a first O2 sensor disposed upstream of the upstream catalyst, a second O2 sensor disposed between the upstream and the downstream catalysts, and a third O2 sensor disposed downstream of the downstream catalyst. Deterioration of each catalyst is determined based on the outputs from these three O2 sensors.
Other conventional methods for determining catalyst deterioration in a single-catalyst converter are described in Patent Application Unexamined Publication (Kokai) No. H2-117890, Kokai No. H5-106494 and Kokai No. H10-61427.
The method disclosed in Kokai No. H2-117890 first performs switching of the fuel correction coefficient at a constant frequency and then calculates integrals or areas in the graphs of the output of the upstream O2 sensor and the downstream O2 sensor to determine the difference of the areas. Deterioration of the catalyst is determined based on the areal difference.
The method disclosed in Kokai No. H5-106494 performs a feedback control of the air-fuel ratio of air-fuel mixture to be supplied to the engine depending on the output of the O2 sensor located on the downstream side of the catalyst disposed in the exhaust pipe. The method uses reversal cycle of the output of the O2 sensor during the feedback control to determine deterioration of the catalyst.
The method disclosed in Kokai No. H10-61427 performs a feedback control of the air-fuel ratio based on the output of the O2 sensor located on the downstream side of the catalyst. The method estimates the oxygen storage capacity of the catalyst based on reversal of the output of the downstream-side O2 sensor and accumulation value of intake airflow. Deterioration of the catalyst is determined based on the estimated oxygen storage capacity.
As shown in the above-referenced Japanese Patent No. 3076417, in order to determine deterioration of the catalysts in the catalyst converter, which includes two or more catalysts in series, a plurality of sensors are provided between each pair of the catalysts disposed in series. This structure incurs extra cost due to increase of the number of the sensors and has a drawback that the control scheme becomes complicated.
If sensors are positioned only on the most upstream and downstream sides of the multiple catalysts in order to resolve the above-stated drawback, only overall deterioration of the multiple catalysts is determined.
Because each catalyst in such a structure having two or more catalysts deteriorates separately depending on the operational conditions as described above, it is desired that deterioration be separately determined for each catalyst.
Therefore, there is a need for a catalyst deterioration detecting system that is capable of separately detecting deterioration of each catalyst utilizing sensors located only on the uppermost stream side and the lowermost stream side in a catalyst converter which includes two or more catalysts in series.